Mr. Vijay Gupta of Kwality Photonics reveling few very important considerations about LED bulbs. Being the 1st LED manufacturer in India Mr. Gupta highlighted the most important factor “heat” and its impact on LED design.
Highly popular 7/9/12 Watt retrofit LED Bulbs with bayonet 2 pin base, that form the core around which the Governments DELP (Domestic Energy Efficient Lighting program) is structured, signal the arrival of that LED technology as a mass technology for good. High volume purchases by ESSL supported ESCO projects in several municipalities across all the states have helped huge capacities in Bulb production come up in the country. Companies like Ecolite, Philips, Fiem, Lemnis, Osram, Crompton, Bajaj, have successfully bid 7watts or 9 Watt bulbs with required efficacy and 2 years guarantee and short delivery times. High volume contracts helped to push down the prices from initial Rs339 all the way down to Rs70 just in a span of 18 months. It’s a clear demonstration of Indian engineering capability that we can match the Chinese in manufacturing skills and process costs, notwithstanding the fact that we have leveraged the lowered cost of inputs facilitated by Chinese vendors. Coming to the technical side of the issue, today’s LED bulbs are evolved afar in design, and literally look like smoothened version of early designs of the A19 LED bulbs. The early version LED bulbs that have been designed with zero base up with as heavy metal mass as possible , with multitude of fins on the body housing, internally containing high power emitters soldered on 1.6mm thick MCPCBs and driven by comprehensively designed LED drivers with no compromise on Large electrolytic capacitors. The prices were in the range of Range of Rs 500 each. The sharp fall in the prices of LEDs since then by 75% abetted by intelligent design compromises have helped Indian LED industry to make a mark on the global LED Scene. Is this the final design that will hold the test of time or will we go back to drawing board once again is the moot point? We can examine if we can use the LED bulb in every possible fixture that housed the incandescent or CFL bulbs. Please note that in the fully enclosed the incandescent bulb fixtures a temperature rise by 100Degrees C hardly effects the functioning of the bulb filament that operated at the 2700C. The incandescent bulb emits most of its waste heat through radiation. Thus even a glass globe will permit much of the IR energy to radiate away from the bulb. But for LEDs such a temperature rise will push the bulb into extinction. There are comments from technical expert that none of the replacement LED bulbs with screw or bayonet bases are really plug and play. They can be successfully used in a few legacy fixtures only. For one the driver electronics requires passive or active cooling which is inadequate in even partially enclosed fixtures, let alone fully enclosed fixtures. LED’s have an inherent drawback in that most of the waste heat is conducted from the chip to its heatsink. Without a means to continue the thermal conduction path to an external heat-sink provided by the fixture, it’ll overheat. The standard base made out of thin sheet metal alone is insufficient. Another important issue is that LED lamps are not dimmer friendly. Either the LED power supply cannot gracefully ramp the current up and down or the conventional dimmer cannot handle the greatly reduced load which does not follow an incandescent lamp resistance curve in the slightest. Making them work properly adds greatly to their expense and can significantly reduce efficiency if a dummy load has to be presented to the power source to help its dimming function. Even more critical is interference from the driver electronics – the RFI fog generated by the switch-mode power supplies used in modern lighting be they solid state fluorescent ballasts or LED lamps. These devices have ruined AM reception in many a home. Even those devices that supposedly meet FCC incidental radiation requirements are too noisy if the receiver’s antenna happens to be within a couple of feet of the lamp fixture. To revisit the thermal issues, if you make a lamp of 800 lumens, using latest generation of chips, you are going to have a prescribed amount of heat period!!! Driver design improvements do not help, as No circuitry is going to improve it by more than a few percent at best. Next, the train has already left the station in terms of perceptions as to how an LED lamp should be designed. In the last 10 years, hundreds companies across the globe have probably spent (wasted?) collectively several hundred million dollars to develop iteration after iteration until we now– finally— have narrowed to a handful of legitimate suppliers and substantial standardization of the basic attributes. A-19 LED bulbs are now selling by the hundreds of thousands per month. 95% of them use a simple buck regulator or a simple transformer-isolated version to drive a string of LEDs. Very little variation among all makers in the basic circuit functionality. Zero chance (or need) at this time for any fundamentally different design scheme, except for the easily fixable thermal issue. The quickest fix is – A thermal cut off circuit with the help of NTC thermistor could do the trick, with small addition to the cost, which at today’s competition scenario, looks un-welcome to the bulb producers. What about active cooling? Majority of mfrs are skeptic about using fans. for one you are adding one more feature that could fail in operation. Even if you were to “blow some air” in there, it is not that simple. Even a fan cooled lamp, base up, in a recessed ceiling fixture, can overheat unless fan can intake cool air and exhaust hot air. Even with a fan, you STILL need protection in event fixture is a configuration with somewhat restricted ability of that “fan” to do its job. Any suggestion like ” just blow some air… transfer heat to enclosure’ however logical, the economics of material cost, assembly cost and dealing with a now viciously competitive market for these 60 W equiv LED lamps, there is little room for extra cost or substantial “re-engineering”. In other words, there may well be all kinds of “elegant” solutions. Today many readymade power extension sockets come with low cost Circuit breakers built-in, unlike 10 years ago. Same will happen to LED bulbs once all consumers understand the value after few repetitive failures and welcome the thermal safety features even at extra cost.
Thermal Design Issues in LED Lights
A typical incandescent bulb emits 3% Light and 97% heat. Filament bulbs emit 12 lumens per every 1 watt of electrical energy input. LEDs are 12 times efficient i.e. 140 lumens per watt, so the light output is 36% of input power. That leaves two thirds power still as heat. Which is huge by any standard? This heat is absorbed by the LED chip, instantly reducing its luminous efficacy and reducing its long term life expectancy significantly. So LEDs need good path for the heat to be transported to the ultimate heat sink – that is – Ambient atmosphere. We LED packaging manufacturers do our first part by adding a metal slug right underneath the internal LED chip. Then the LED lamp makers connect the het path to Copper track on the MPCB using thermal interface materials, and onwards to the metal body which acts as final heatsinks with multiple fins t increase convection losses of heat.
How does heat affect LED lighting?
As the junction temperature (Tj) of an LED increases, multiple performance parameters are compromised. QUALITY of light is affected through color shift and white point instability. QUANTITY of light is lowered whilst using the same power, decreasing the energy efficiency. LIFE is also decreased through accelerated lumen depreciation generating poor return on your investment in LED technology.
